Why can't I see any transparency on my material with Eevee?
First of all with Principled BSDF you don't have to mix it with Transparent because it already has its own transparency mixer, Alpha slider. The second - in material tab in settings make sure your Blend mode is 'Alpha Blend' and 'Show Backface' is enabled.

UPD. 'Show Backface' option indeed shows the back sides of the polygons but simultaneously stops to show the front sides. I guess in some cases it can be used as a kind of fake semi-transparency, but in fact it is not.